Re: [RADIR] draft-narten-radir-problem-statement

Phil Roberts <roberts@isoc.org> Tue, 16 February 2010 16:56 UTC

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Date: Tue, 16 Feb 2010 10:57:52 -0600
From: Phil Roberts <roberts@isoc.org>
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Cc: Olaf Kolkman <olaf@NLnetLabs.nl>, chair@ietf.org, radir@ietf.org
Subject: Re: [RADIR] draft-narten-radir-problem-statement
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You're welcome.   Any idea about when this will go to last call and get 
published?


Thomas Narten wrote:
> Hi Phil. Thanks for the comments.
>
> Below is a new version that incorporates all but one of your
> suggestions.
>
>   
>> One might consider whether to rename "Business considerations" into 
>> "Operational considerations?"  There certainly are business 
>> considerations but there are also operational policy issues worked in 
>> that are important.   I'm not saying this because I balk at business 
>> considerations in docs like this, though others may complain about that 
>> also.
>>     
>
> I didn't make this change. After re-reading the section, I guess I'm
> not convinced that "operational considerations" is really more
> accurate than "Business Considerations".
>
> I also took Russ' suggestion and pretty much excised the word
> "problem".  
>
> That included changing the title from "Routing and Addressing Problem
> Statement" to "On the Scalability of Internet Routing".  I had a
> longer title I liked better, but xml2rfc barfed on it and it took me
> 20 minutes to figure out where the problem was. :-(
>
> I also reread it carefully and tweaked some of the wording and got
> some updated numbers from the CIDR reports.
>
> One question for radir:
>
>   
>>    For a large ISP, the internal IPv4 table can be between 50,000 and
>>    150,000 routes.  During the dot com boom some ISPs had more internal
>>    prefixes than there were in the Internet table.  Thus the size of the
>>    internal routing table can have significant impact on the scalability
>>    and should not be discounted.
>>     
>
> Is that 50-150K number still right? Or should it be tweaked?
>
> I plan on posting the document Monday, but if anyone wants more time
> to review first just say so. There is a discusion on rrg this week
> where pointing them to the document would be timely.
>
> I think the document is in pretty good shape. The only change I still
> have on my TODO list is clean up some of the references.
>
> Thomas
>
>
>
>
>
> Network Working Group                                          T. Narten
> Internet-Draft                                                       IBM
> Intended status: Informational                         February 12, 2010
> Expires: August 16, 2010
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>                  On the Scalability of Internet Routing
>               draft-narten-radir-problem-statement-05.txt
>
> Abstract
>
>    There has been much discussion over the last years about the overall
>    scalability of the Internet routing system.  Some have argued that
>    the resources required to maintain routing tables in the core of the
>    Internet are growing faster than available technology will be able to
>    keep up.  Others disagree with that assessment.  This document
>    attempts to describe the factors that are placing pressure on the
>    routing system and the growth trends behind those factors.
>
> Status of this Memo
>
>    This Internet-Draft is submitted to IETF in full conformance with the
>    provisions of BCP 78 and BCP 79.
>
>    Internet-Drafts are working documents of the Internet Engineering
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>    This Internet-Draft will expire on August 16, 2010.
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> Copyright Notice
>
>    Copyright (c) 2010 IETF Trust and the persons identified as the
>    document authors.  All rights reserved.
>
>    This document is subject to BCP 78 and the IETF Trust's Legal
>
>
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> Narten                   Expires August 16, 2010                [Page 1]
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> Table of Contents
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>    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
>    2.  Terms and Definitions  . . . . . . . . . . . . . . . . . . . .  4
>    3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
>      3.1.  Technical Aspects  . . . . . . . . . . . . . . . . . . . .  7
>      3.2.  Business Considerations  . . . . . . . . . . . . . . . . .  7
>      3.3.  Alignment of Incentives  . . . . . . . . . . . . . . . . .  8
>      3.4.  Table Growth Targets . . . . . . . . . . . . . . . . . . .  9
>    4.  Pressures on Routing Table Size  . . . . . . . . . . . . . . . 10
>      4.1.  Traffic Engineering  . . . . . . . . . . . . . . . . . . . 10
>      4.2.  Multihoming  . . . . . . . . . . . . . . . . . . . . . . . 11
>      4.3.  End Site Renumbering . . . . . . . . . . . . . . . . . . . 12
>      4.4.  Acquisitions and Mergers . . . . . . . . . . . . . . . . . 12
>      4.5.  RIR Address Allocation Policies  . . . . . . . . . . . . . 12
>      4.6.  Dual Stack Pressure on the Routing Table . . . . . . . . . 13
>      4.7.  Internal Customer Routes . . . . . . . . . . . . . . . . . 14
>      4.8.  IPv4 Address Exhaustion  . . . . . . . . . . . . . . . . . 14
>    5.  Pressures on Control Plane Load  . . . . . . . . . . . . . . . 15
>      5.1.  Interconnection Richness . . . . . . . . . . . . . . . . . 15
>      5.2.  Multihoming  . . . . . . . . . . . . . . . . . . . . . . . 15
>      5.3.  Traffic Engineering  . . . . . . . . . . . . . . . . . . . 15
>      5.4.  Questionable Operational Practices?  . . . . . . . . . . . 16
>        5.4.1.  Rapid shuffling of prefixes  . . . . . . . . . . . . . 16
>        5.4.2.  Anti-Route Hijacking . . . . . . . . . . . . . . . . . 16
>        5.4.3.  Operational Ignorance  . . . . . . . . . . . . . . . . 16
>      5.5.  RIR Policy . . . . . . . . . . . . . . . . . . . . . . . . 17
>    6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
>    7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
>    8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
>    9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
>    10. Informative References . . . . . . . . . . . . . . . . . . . . 23
>    Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
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> 1.  Introduction
>
>    Prompted in part by the October, 2006 IAB workshop on Routing &
>    Addressing [RFC4984], there has been a renewed focus on the topic of
>    routing scalability within the Internet.  The issue itself is not
>    new, with discussions dating back at least 10-15 years [GSE, ROAD].
>
>    This document attempts to describe the "pain points" being placed on
>    the routing system, with the aim of describing the essential aspects
>    so that the community has a way of evaluating whether proposed
>    changes to the routing system actually address or impact existing
>    pain points in a significant manner.
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> 2.  Terms and Definitions
>
>    Control Plane:  The routing setup protocols, their associated state
>       and the activity needed to create and maintain the data structures
>       used to forward packets from one network to another.  The term is
>       defined broadly to include all protocols and activities needed to
>       construct and maintain the forwarding tables used to forward
>       packets.
>
>    Control Plane Load:  The actual load associated with operating the
>       Control Plane.  The higher the control plane load, the higher the
>       cost of operating the control plane (in terms of hardware,
>       bandwidth, power, etc.).  The terms "routing load" and "control
>       plane load" are used interchangeably throughout this document.
>
>    Control Plane Cost:  The overall cost associated with operating the
>       Control Plane.  The cost consists of capital costs (for hardware),
>       bandwidth costs (for the control plane signalling) and any other
>       ongoing operational cost associated with operating and maintaining
>       the control plane.
>
>    Default Free Zone (DFZ):  That part of the Internet where routers
>       maintain full routing tables.  Many routers maintain only partial
>       tables, having explicit routes for "local" destinations (i.e.,
>       prefixes) plus a "default" for everything else.  For such routers,
>       building and maintaining routing tables is relatively simple
>       because the amount of information learned and maintained can be
>       small.  In contrast, routers in the DFZ maintain complete
>       information about all reachable destinations, which at the time of
>       this writing number in the hundreds of thousands of entries.
>
>    Routing Information Base (RIB):  The data structures a router
>       maintains that hold the information about destinations and paths
>       to those destinations.  The amount of state information maintained
>       is dependent on a number of factors, including the number of
>       individual prefixes learned from peers, the number of BGP peers,
>       the number of distinct paths interconnecting destinations, etc.
>       In addition to maintaining information about active paths used for
>       forwarding, the RIB may also include information about unused
>       ("backup") paths.
>
>    Forwarding Information Base (FIB):  The actual table consulted while
>       making forwarding decisions for individual packets.  The FIB is a
>       compact, optimized subset of the RIB, containing only the
>       information needed to actually forward individual packets, i.e.,
>       mapping a packet's destination address to an outgoing interface
>       and next-hop.  The FIB only stores information about paths
>       actually used for forwarding; it typically does not store
>
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>       information about backup paths.  The FIB is typically constructed
>       from specialized hardware components, which have different (and
>       higher) cost properties than the hardware typically used to
>       maintain the RIB.
>
>    Traffic Engineering (TE):  In this document, "traffic engineering"
>       refers to the current practice of inbound, inter-AS traffic
>       engineering.  TE is accomplished by injecting additional, more-
>       specific routes into the routing system and/or increasing the
>       frequency of routing updates in order to arrange for inbound
>       traffic at the boundary of an Autonomous system (AS) to travel
>       over a different path than it otherwise would.
>
>    Provider Aggregatable (PA) address space:  Address space that an end
>       site obtains from an upstream ISP's address block.  The main
>       benefit of PA address space is that reachability to all of a
>       provider's customers can be achieved by advertising a single
>       "provider aggregate" address prefix into the DFZ, rather than
>       needing to announce individual prefixes for each customer.  An
>       important disadvantage is that when a customer changes providers,
>       the customer must renumber their site into addresses belonging to
>       the new provider and return the previously used addresses to the
>       former provider.
>
>    Provider Independent (PI) address space:  Address space that an end
>       site obtains directly from a Regional Internet Registry (RIR) for
>       addressing its devices.  The main advantage (for the end site) is
>       that it does not need to renumber its site when changing
>       providers, since it continues to use its PI block.  However, PI
>       address blocks are not aggregatable and thus each individual PI
>       assignment results in an individual prefix being injected into the
>       DFZ.
>
>    Site:  Any topologically and administratively distinct entity that
>       connects to the Internet.  A site can range from a large
>       enterprise or ISP to a small home site.
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> 3.  Background
>
>    Within the DFZ, both the size of the RIB and FIB and the overall
>    update rate have historically increased at a greater than linear
>    rate.  Specifically:
>
>    o  The number of individual prefixes that are being propagated into
>       the DFZ over time has been and continues to increase at a faster-
>       than-linear rate.  The term "super-linear" has been used to
>       characterize the growth.  The exact nature of the growth is much
>       debated (e.g., quadratic, polynomial, etc.), but growth is clearly
>       faster than linear.  The reasons behind the rate increase are
>       varied and discussed below.  Because each individual prefix
>       requires resources to process, any increase in the number of
>       prefixes produces a corresponding increase in control plane load
>       of the routing system.  Each individual prefix that appears in
>       routing updates requires state in the RIB (and possibly the FIB)
>       and consumes processing and other resources when updates related
>       to the prefix are received.
>
>    o  The overall rate of routing updates is increasing [1], requiring
>       routers to process updates at an increased rate or converge more
>       slowly if they cannot.  The rate increase of the control plane
>       load is driven by a number of factors (discussed below).  Further
>       study is needed to better understand the factors behind the
>       increasing update rate.  For example, it appears that a
>       disproportionate increase in observed updates originates from a
>       small percentage of the total number of advertised prefixes.
>
>    The super-linear growth in the routing load presents a scalability
>    challenge for current and/or future routers.  While there appears to
>    be general agreement that we will be able to build routers (i.e.,
>    hardware & software) actually capable of handling the control plane
>    load, both today and going forward, there is considerable debate
>    about the cost.  In particular, will it be possible for ISPs that
>    currently (or would like to) maintain routes as part of the DFZ be
>    able to afford to do so, or will only the largest (and a shrinking
>    number) of top tier ISPs be able to afford the investment and cost of
>    operating the control plane while being a part of the DFZ?
>
>    Finally, the scalability challenge is aggravated by the lack of any
>    firm limiting architectural upper-bound on the growth rate of the
>    routing load and a weakening of social constraints that historically
>    have helped restrain the growth rate so far.  Going forward, there is
>    considerable uncertainty (some would say doubt) whether future growth
>    rates will continue to be sufficiently constrained so that router
>    development can keep up at an acceptable price point.
>
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> 3.1.  Technical Aspects
>
>    The technical challenge of building routers relates to the resources
>    needed to process a larger and increasingly dynamic amount of routing
>    information.  More specifically, routers must maintain an increasing
>    amount of associated state information in the RIB, they must be
>    capable of populating a growing FIB, they must perform forwarding
>    lookups at line rates (while accessing the FIB) and they must be able
>    to initialize the RIB and FIB after system restart.  All of these
>    activities must take place within acceptable time frames (i.e., paths
>    for individual destinations must converge and stabilize within an
>    acceptable time period).  Finally, the hardware needed to achieve
>    this cannot have unreasonable power consumption or cooling demands.
>
> 3.2.  Business Considerations
>
>    While the IETF does not (and cannot) concern itself with business
>    models or the profitability of the ISP community, the cost of running
>    the routing subsystem as a whole is directly influenced by the
>    routing architecture of the Internet, which clearly is the IETF's
>    business.  Thus, it is useful to consider the overall business
>    environment that underlies operation of the DFZ routing
>    infrastructure.  The DFZ is run entirely by the private sector with
>    no overall governmental oversight or regulatory framework to oversee
>    or even influence what routes are propagated where, who must carry
>    them, etc.  ISPs decide (on their own) which routing updates to
>    accept and how (if at all) to process them.  Thus, there is no
>    overall authority that can limit the number of prefixes that are
>    injected into the DFZ or that insure that any particular prefixes are
>    accepted at all.  Today, the system functions because the set of
>    entities that comprise the DFZ are (generally) able to accept the
>    prefixes that are being advertised and some loose best practices have
>    emerged that are generally followed (e.g., minimum prefix sizes that
>    are routed coupled with RIR policies that place limitations on who
>    may obtain PI prefixes).
>
>    In general the Internet would benefit if the cost of the (routing)
>    infrastructure did not grow too rapidly as the Internet grows, since
>    a lower infrastructure cost makes it possible to provide Internet
>    service at a lower cost to a larger number of users.  That said, some
>    types of Internet growth tie directly to revenue opportunities or
>    cost savings for an ISP (e.g., adding more users/customers,
>    increasing bandwidth, technological advances, providing new or
>    additional services, etc.).  Upgrading or changing infrastructure is
>    most feasible (and expected) when supported by a workable cost
>    recovery model.  Hence limiting the cost of self-induced scaling is a
>    nice-to-have benefit, but not a requirement.
>
>
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>    On the other hand, it is problematic when the infrastructure cost for
>    an ISP grows (rapidly) due to factors outside of its own control,
>    e.g., resulting from overall Internet growth external to the ISP.  If
>    an ISP that does not add new customers, upgrade the bandwidth for
>    their customers, or provide new services needs to upgrade or replace
>    their infrastructure in unexpected ways, then they have no natural
>    cost recovery mechanisms.  This is in essence what is happening with
>    the scaling of the global routing table.  An ISP that is part of the
>    DFZ may need to upgrade its routers to handle an increased routing
>    load just to maintain the same level of service with respect to their
>    current customers and services.
>
>    Even if it is technically possible to build routers capable of
>    meeting the technical and operational requirements, it is also
>    necessary that the overall cost to build, maintain and deploy such
>    equipment meet reasonable business expectations.  ISPs, after all,
>    are run as businesses.  As such, they must be able to plan, develop
>    and construct viable business plans that provide an acceptable return
>    on investment (i.e., one acceptable to investors).
>
> 3.3.  Alignment of Incentives
>
>    Today's growth pattern is influenced by the scaling properties of the
>    current routing system.  If the routing system had better scaling
>    properties, we would be able support and enable more widespread usage
>    of such services as multihoming and traffic engineering.  The current
>    system simply would not be able to handle to the routing load if
>    everyone were to choose to multihome.  There are millions of
>    potential end sites that would benefit from being able to multihome.
>    This compares with a low few hundred thousand prefixes being carried
>    today.  Broader availability of multihoming is limited by barriers
>    imposed by operational practices that try to strike a balance between
>    the amount of multihoming and preservation of routing slots.  It is
>    desirable that the routing and addressing system exert the least
>    possible back pressure on end user applications and deployment
>    scenarios, to enable the broadest possible use of the Internet.
>
>    One aspect of the current architecture is a misalignment of cost and
>    benefit.  Injecting individual prefixes into the DFZ creates a small
>    amount of "pain" for those routers that are part of the DFZ.  Each
>    individual prefix adds only a small cost to the routing load, but the
>    aggregate sum of all prefixes is significant, and leads to the key
>    issue at hand.  Those that inject prefixes into the DFZ do not
>    generally pay the cost associated with the individual prefix -- it is
>    carried by the routers in the DFZ.  But the originator of the prefix
>    receives the benefit.  Hence, there is misalignment of incentives
>    between those receiving the benefit and those bearing the cost of
>    providing the benefit.  Consequently, incentives are not aligned
>
>
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>    properly to produce a natural feedback loop to balance the cost and
>    benefit of maintaining routing tables.
>
> 3.4.  Table Growth Targets
>
>    A precise target for the rate of table size or routing update
>    increase that should reasonably be supported going forward is
>    difficult to state in quantitative terms.  One target might simply be
>    to keep the growth at a stable, but manageable growth rate so that
>    the increased router functionality can roughly be covered by
>    improvements in technology (e.g., increased processor speeds,
>    reductions in component costs, etc.).
>
>    However, it is highly desirable to significantly bring down (or even
>    reverse) the growth rate in order to meet user expectations for
>    specific services.  As discussed below, there are numerous pressures
>    to deaggregate routes.  These pressures come from users seeking
>    specific, tangible service improvements that provide "business-
>    critical" value.  Today, some of those services simply cannot be
>    supported to the degree that future demand can reasonably be expected
>    because of the negative implications on DFZ table growth.  Hence,
>    valuable services are available to some, but not all potential
>    customers.  As the need for such services becomes increasingly
>    important, it will be difficult to deny such services to large
>    numbers of users, especially when some "lucky" sites are able to use
>    the service and others are not.
>
>
>
>
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> 4.  Pressures on Routing Table Size
>
>    There are a number of factors behind the increase in the quantity of
>    prefixes appearing in the DFZ.  From a theoretical perspective, the
>    number of prefixes in the DFZ can be minimized through aggressive
>    aggregation [RFC4632].  In practice, strict adherence to the CIDR
>    principles is difficult.
>
> 4.1.  Traffic Engineering
>
>    Traffic engineering (TE) is the act of arranging for certain Internet
>    traffic to use or avoid certain network paths (that is, TE attempts
>    to place traffic where capacity exists, or where some set of
>    parameters of the path is more favorable to the traffic being placed
>    there).
>
>    Outbound TE is typically accomplished by using internal interial
>    gateway protocol (IGP) metrics to choose the shortest exit for two
>    equally good BGP paths.  Adjustment of IGP metrics controls how much
>    traffic flows over different internal paths to specific exit points
>    for two equally good BGP paths.  Additional traffic can be moved by
>    applying some policy to depreference or filter certain routes from
>    specific BGP peers.  Because outbound TE is achieved via a site's own
>    IGP, outbound TE does not impact routing outside of a site.
>
>    Inbound TE is performed by announcing a more-specific route along the
>    preferred path that "catches" the desired traffic and channels it
>    away from the path it would take otherwise (i.e., via a larger
>    aggregate).  At the BGP level, if the address range requiring TE is a
>    portion of a larger address aggregate, network operators implementing
>    TE are forced to de-aggregate otherwise aggregatable prefixes in
>    order to steer the traffic of the particular address range to
>    specific paths.
>
>    TE is performed by both ISPs and customer networks, for three primary
>    reasons:
>
>    o  to match traffic with network capacity, or to spread the traffic
>       load across multiple links (frequently referred to as "load
>       balancing")
>
>    o  to reduce costs by shifting traffic to lower cost paths or by
>       balancing the incoming and outgoing traffic volume to maintain
>       appropriate peering relations
>
>    o  to enforce certain forms of policy (e.g., to prevent government
>       traffic from transiting through other countries)
>
>
>
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>    TE impacts route scaling in two ways.  First, inbound TE can result
>    in additional prefixes being advertised into the DFZ.  Second,
>    Network operators usually achieve traffic engineering by "tweaking"
>    the processing of routing protocols to achieve desired results, e.g.,
>    by sending updates at an increased rate.  In addition, some devices
>    attempt to automatically find better paths and then advertise those
>    preferences through BGP, though the extent to which such tools are in
>    use and contributing to the control plane load is unknown.
>
>    In today's highly competitive environment, providers require TE to
>    maintain good performance and low cost in their networks.
>
> 4.2.  Multihoming
>
>    Multihoming refers generically to the case in which a site is served
>    by more than one ISP [RFC4116].  Multihoming is used to provide
>    backup paths (i.e., to remove single points of failure), to achieve
>    load-sharing, and to achieve policy or performance objectives (e.g.,
>    to use lower latency or higher bandwidth paths).  Multihoming may
>    also be a requirement due to contract or law.
>
>    Multihoming can be accomplished using either PI or PA address space.
>    A multihomed site advertises its site prefix into the routing system
>    of each of its providers.  For PI space, the site's PI space is used,
>    and the prefix is propagated throughout the DFZ.  For PA space, the
>    PA site prefix may (or may not) be propagated throughout the DFZ,
>    with the details depending on what type of multihoming is sought.
>
>    If the site uses PA space, the PA site prefix allocated from one of
>    its providers (whom we'll call the Primary Provider) is used.  The PA
>    site prefix will be aggregatable by the Primary Provider but not the
>    others.  To achieve multihoming with comparable properties to that
>    when PI addresses are used as described above, the PA site prefix
>    will need to be injected into the routing system of all of its ISPs,
>    and throughout the DFZ.  In addition, because of the longest-match
>    forwarding rule, the Primary Provider must advertise both its
>    aggregate and the individual PA site prefix; otherwise, the path via
>    the primary provider (as advertised via the aggregate) will never be
>    selected due to the longest match rule.  For the type of multihoming
>    described here, where the PA site prefix is propagated throughout the
>    DFZ, the use of PI vs. PA space has no impact on the control plane
>    load.  The increased load is due entirely to the need to propagate
>    the site's individual prefix throughout the DFZ.
>
>    The demand for multihoming is increasing [2].  The increase in
>    multihoming demand is due to the increased reliance on the Internet
>    for mission and business-critical applications (where businesses
>    require 7x24 availability for their services) and the general
>
>
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>    decrease in cost of Internet connectivity.
>
> 4.3.  End Site Renumbering
>
>    It is generally considered painful and costly to renumber a site,
>    with the cost proportional to the size and complexity of the network
>    and most importantly, to the degree that addresses are stored in
>    places that are difficult in practice to update.  When using PA
>    space, a site must renumber when changing providers.  Larger sites
>    object to this cost and view the requirement to renumber akin to
>    being held "hostage" to the provider from which PA space was
>    obtained.  Consequently, many sites desire PI space.  Having PI space
>    provides independence from any one provider and makes it easier to
>    switch providers (for whatever reason).  However, each individual PI
>    prefix must be propagated throughout the DFZ and adds to the control
>    plane load.
>
>    It should be noted that while larger sites may also want to
>    multihome, the cost of renumbering drives some sites to seek PI
>    space, even though they do not multihome.
>
> 4.4.  Acquisitions and Mergers
>
>    Acquisitions and mergers take place for business reasons, which
>    usually have little to do with the network topologies of the impacted
>    organizations.  When a business sells off part of itself, the assets
>    may include networks, attached devices, etc.  A company that
>    purchases or merges with other organizations may quickly find that
>    its network assets are numbered out of many different and
>    unaggragatable address blocks.  Consequently, an individual
>    organization may find itself unable to announce a single prefix for
>    all of their networks without renumbering a significant portion of
>    its network.
>
>    Likewise, selling off part of a business may involve selling part of
>    a network as well, resulting in the fragmentation of one address
>    block into two (or more) smaller blocks.  Because the resultant
>    blocks belong to different organizations, they can no longer be
>    advertised by a single aggregate and the resultant fragments may need
>    to be advertised individually into the DFZ.
>
> 4.5.  RIR Address Allocation Policies
>
>    ISPs and multihoming end sites obtain address space from RIRs.  As an
>    entity grows, it needs additional address space and requests more
>    from its RIR.  In order to be able to obtain additional address space
>    that can be aggregated with the previously-allocated address space,
>    the RIR must keep a reserve of space that the requester can grow into
>
>
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>    in the future.  But any reserved address space cannot be used for any
>    other purpose (i.e., assigned to another organization).  Hence, there
>    is an inherent conflict between holding address space in reserve to
>    allow for the future growth of an existing allocation holder and
>    using address space efficiently.  In IPv4, there has been a heavy
>    emphasis on conserving address space and obtaining efficient
>    utilization.  Consequently, insufficient space has been held in
>    reserve to allow for the growth of all sites and some allocations
>    have had to be made from discontiguous address blocks.  That is, some
>    sites have received discontiguous address blocks because their growth
>    needs exceeded the amount of space held in reserve for them.
>
>    In IPv6, its vast address space allows for a much a greater emphasis
>    to be placed placed on preserving future aggregation than was
>    possible in IPv4.
>
> 4.6.  Dual Stack Pressure on the Routing Table
>
>    The recommended IPv6 deployment model is dual-stack, where IPv4 and
>    IPv6 are run in parallel across the same links.  This has two
>    implications for routing.  First, although alternative scenarios are
>    possible, it seems likely that many routers will be supporting both
>    IPv4 and IPv6 simultaneously and will thus be managing both IPv4 and
>    IPv6 routing tables within a single router.  Second, for sites
>    connected via both IPv4 and IPv6, both IPv4 and IPv6 prefixes will
>    need to be propagated into the routing system.  Consequently, dual-
>    stack routers will maintain both an IPv4 and IPv6 route to reach the
>    same destination.
>
>    It is possible to make some simple estimates on the approximate size
>    of the IPv6 tables that would be needed if all sites reachable via
>    IPv4 today were also reachable via IPv6.  In theory, each autonomous
>    system (AS) needs only a single aggregate route.  This provides a
>    lower bound on the size of the fully-realized IPv6 routing table.
>    (As of Feb 2010, [3] states there are 33,548 active ASes in the
>    routing system.)
>
>    A single IPv6 aggregate will not allow for inbound traffic
>    engineering.  End sites will need to advertise a number of smaller
>    prefixes into the DFZ if they desire to gain finer grained control
>    over their IPv6 inbound traffic.  This will increase the size of the
>    IPv6 routing table beyond the lower bound discussed above.  There is
>    reason to expect the IPv6 routing table will be smaller than the
>    current IPv4 table, however, because the larger initial assignments
>    to end sites will minimize the de-aggregation that occurs when a site
>    must go back to its upstream address provider or RIR and receive a
>    second, non-contiguous assignment.
>
>
>
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>    It is possible to extrapolate what the size of the IPv6 Internet
>    routing table would be if widespread IPv6 adoption occurred, from the
>    current IPv4 Internet routing table.  Each active AS (33,548) would
>    require at least one aggregate.  In addition, the IPv6 Internet table
>    would also carry more-specific prefixes for traffic engineering.
>    Assume that the IPv6 Internet table will carry the same number of
>    more specifics as the IPv4 Internet table.  In this case one can take
>    the number of IPv4 Internet routes and subtract the number of CIDR
>    aggregates that they could easily be aggregated down to.  As of Feb
>    2010, the 313,626 routes can be easily aggregated down to 193,844
>    CIDR aggregates [3].  That difference yields 119,782 extra more-
>    specific prefixes.  Thus if each active AS (33,548) required one
>    aggregate, and an additional 119,782 more specifics were required,
>    then the IPv6 Internet table would be 153,330 prefixes.
>
> 4.7.  Internal Customer Routes
>
>    In addition to the Internet routing table, networks must also support
>    their internal routing table.  Internal routes are defined as more-
>    specific routes that are not advertised to the DFZ.  This primarily
>    consists of prefixes that are a more-specific of a provider aggregate
>    (PA) and are assigned to a single-homed customer.  The DFZ need only
>    carry the PA aggregate in order to deliver traffic to the provider.
>    However, the provider's routers require the more-specific route to
>    deliver traffic to the end site.
>
>    Internal routes could also come from more-specific prefixes
>    advertised by multihomed customers with the "no-export" BGP
>    community.  This is useful when the fine grained control of traffic
>    to be influenced can be contained to the neighboring network.
>
>    For a large ISP, the internal IPv4 table can be between 50,000 and
>    150,000 routes.  During the dot com boom some ISPs had more internal
>    prefixes than there were in the Internet table.  Thus the size of the
>    internal routing table can have significant impact on the scalability
>    and should not be discounted.
>
> 4.8.  IPv4 Address Exhaustion
>
>    The IANA and RIR free pool of IPv4 addresses will be exhausted within
>    a few years.  As the free pool shrinks, the size of the remaining
>    unused blocks will also shrink and unused blocks previously held in
>    reserve for expansion of existing allocations or otherwise not used
>    due to their smaller size will be allocated for use.  Consequently,
>    as the community looks to use every piece of available address space
>    (no matter how small) there will be an increasing pressure to
>    advertise additional prefixes in the DFZ.
>
>
>
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> 5.  Pressures on Control Plane Load
>
>    This section describes a number of trends and pressures that are
>    contributing to the overall routing load.  The previous section
>    described pressures that are increasing the size of the routing
>    table.  Even if the size could be bounded, the amount of work needed
>    to maintain paths for a given set of prefixes appears to be
>    increasing.
>
> 5.1.  Interconnection Richness
>
>    The degree of interconnectedness between ASes has increased in recent
>    years.  That is, the Internet as a whole is becoming "flatter" with
>    an increasing number of possible paths interconnecting sites [4].  As
>    the number of possible paths increase, the amount of computation
>    needed to find a best path also increases.  This computation comes
>    into effect whenever a change in path characteristics occurs, whether
>    from a new path becoming available, an existing path failing, or a
>    change in the attributes associated with a potential path.  Thus,
>    even if the total number of prefixes were to stay constant, an
>    increase in the interconnection richness implies an increase in the
>    resources needed to maintain routing tables.
>
> 5.2.  Multihoming
>
>    Multihoming places pressure on the routing system in two ways.
>    First, an individual prefix for a multihomed site (whether PI or PA)
>    must be propagated into the routing system, so that other sites can
>    find a good path to the site.  Even if the site's prefix comes out of
>    a PA block, an individual prefix for the site needs to be advertised
>    so that the most desirable path to the site can be chosen when the
>    path through the aggregate is sub-optimal.  Second, a multihomed site
>    will be connected to the Internet in more than one place, increasing
>    the overall level of interconnection richness.  If an outage occurs
>    on any of the circuits connecting the site to the Internet, those
>    changes will be propagated into the routing system.  In contrast, a
>    singly-homed site numbered out of a Provider Aggregate places no
>    additional control plane load in the DFZ as the details of the
>    connectivity status to the site are kept internal to the provider to
>    which it connects.
>
> 5.3.  Traffic Engineering
>
>    The mechanisms used to achieve multihoming and inbound Traffic
>    Engineering are the same.  In both cases, a specific prefix is
>    advertised into the routing system to "catch" traffic and route it
>    over a different path than it would otherwise be carried.  When
>    multihoming, the specific prefix is one that differs from that of its
>
>
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>    ISP or is a more-specific of the ISP's PA.  Traffic Engineering is
>    achieved by taking one prefix and dividing it into a number of
>    smaller and more-specific ones, and advertising them in order to gain
>    finer-grained control over the paths used to carry traffic covered by
>    those prefixes.
>
>    Traffic Engineering increases the number of prefixes carried in the
>    routing system.  In addition, when a circuit fails (or the routing
>    attributes associated with the circuit change), additional load is
>    placed on the routing system by having multiple prefixes potentially
>    impacted by the change, as opposed to just one.
>
> 5.4.  Questionable Operational Practices?
>
>    Some operators are believed to engage in operational practices that
>    increase the load on the routing system.
>
> 5.4.1.  Rapid shuffling of prefixes
>
>    Some networks try to assert fine-grained control of inbound traffic
>    by modifying route announcements frequently in order to migrate
>    traffic to less loaded links quickly.  The goal of this is to achieve
>    higher utilization of multiple links.  In addition, some route
>    selection devices actively measure link or path utilization and
>    attempt to optimize inbound traffic by withholding or depreferencing
>    certain prefixes in their advertisements.  In short, any system that
>    actively measures load and modifies route advertisements in real time
>    increases the load on the routing system, as any change in what is
>    advertised must ripple through the entire routing system.
>
> 5.4.2.  Anti-Route Hijacking
>
>    In order to reduce the threat of accidental (or intentional)
>    hijacking of its address space by an unauthorized third party, some
>    sites advertise their space as a set of smaller prefixes rather than
>    as one aggregate.  That way, if someone else advertised a path for
>    the larger aggregate (or a small piece of the aggregate), it will be
>    ignored in favor of the more-specific announcements.  This increases
>    both the number of prefixes advertised, and the number of updates.
>
> 5.4.3.  Operational Ignorance
>
>    It is believed that some undesirable practices result from operator
>    ignorance, where the operator is unaware of what they are doing and
>    the impact that has on the DFZ.
>
>    The default behavior of most BGP configurations is to automatically
>    propagate all learned routes.  That is, one must take explicit
>
>
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>    configuration steps to prevent the automatic propagation of learned
>    routes.  In addition, it is often significant work to figure out how
>    to (safely) aggregate routes (and which ones to aggregate) in order
>    to reduce the number of advertisements propagated elsewhere.  While
>    vendors could provide additional configuration "knobs" to reduce
>    leakage, the implementation of additional features increases
>    complexity and some operators may fear that the new configuration
>    will break their existing routing setup.  Finally, leaking routes
>    unnecessarily does not generally harm those responsible for the
>    misconfiguration, hence, there may be little incentive to change such
>    behavior.
>
> 5.5.  RIR Policy
>
>    RIR address policy has direct impact on the control plane load
>    because address policy determines who is eligible for a PI assignment
>    (which impacts how many are given out in practice) and the size of
>    the assignment (which impacts how much address space can be
>    aggregated within a single assignment).  If PI assignments for end
>    sites did not exist, then those end sites would not advertise their
>    own prefix directly into the global routing system; instead their
>    address block would be covered by their provider's aggregate.  That
>    said, RIRs have adopted PI policies in response to community demand,
>    for reasons described elsewhere (e.g., to support multihoming and to
>    avoid the need to renumber).  In short, RIR policy can be seen as a
>    symptom rather than a root cause.
>
>
>
>
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> 6.  Summary
>
>    As discussed in previous sections, in the current operating
>    environment, an ISP may experience an overall increase in the routing
>    load due entirely to external factors outside of its control.  These
>    external pressures can make it increasingly difficult for ISPs to
>    recover control plane related costs associated with the growth of the
>    Internet.  Moreover, real business and user needs are creating
>    increasing pressure to use techniques that increase the control plane
>    load for ISPs operating within the DFZ.  While the system largely
>    works today, there is a real risk that the current cost and incentive
>    structures will be unable to keep control plane costs manageable
>    (within the context of then-available routing hardware) over the next
>    decades.  The Internet would strongly benefit from a routing and
>    addressing model designed with this in mind.  Thus, in the absence of
>    a business model that better supports such cost recovery, there is a
>    need for an approach to routing and addressing that fulfils the
>    following criteria:
>
>    1.  Provides sufficient benefits to the party bearing the costs of
>        deploying and maintaining the technology to recover the cost for
>        doing so.
>
>    2.  Reduces the growth rate of the DFZ control plane load.  In the
>        current architecture, this is dominated by the routing, which is
>        dependent on:
>
>        A.  The number of individual prefixes in the DFZ
>
>        B.  The update rate associated with those prefixes.
>
>        Any change to the control plane architecture must result in a
>        reduction in the overall control plane load, and shouldn't simply
>        shift the load from one place in the system to another, without
>        reducing the overall load as a whole.
>
>    3.  Allows any end site wishing to multihome to do so
>
>    4.  Supports ISP and enterprise TE needs
>
>    5.  Allows end sites to switch providers while minimizing
>        configuration changes to internal end site devices.
>
>    6.  Provides end-to-end convergence/restoration of service at least
>        comparable to that provided by the current architecture
>
>    This document has purposefully been scoped to focus on the growth of
>    the routing control plane load of operating the DFZ.  Other problems
>
>
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>    that may seem related, but do not directly impact on route scaling
>    are not considered to be "in scope" at this time.  For example,
>    Mobile IP [RFC3344] [RFC3775] and NEMO [RFC3963] place no pressures
>    on the routing system.  They are layered on top of existing IP, using
>    tunneling to forward packets via a care-of addresses.  Hence,
>    "improving" these technologies (e.g., by having them leverage a
>    solution to the multihoming problem), while a laudable goal, is not
>    considered a necessary goal.
>
>
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> 7.  Security Considerations
>
>    This document does not introduce any security considerations.
>
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> 8.  IANA Considerations
>
>    This document contains no IANA actions.
>
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> 9.  Acknowledgments
>
>    The initial version of this document was produced by the Routing and
>    Addressing Directorate (http://www.ietf.org/IESG/content/radir.html).
>    The membership of the directorate at that time included Marla
>    Azinger, Vince Fuller, Vijay Gill, Thomas Narten, Erik Nordmark,
>    Jason Schiller, Peter Schoenmaker, and John Scudder.
>
>    Comments should be sent to rrg@iab.org or to radir@ietf.org.
>
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> 10.  Informative References
>
>    [RFC3344]  Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
>               August 2002.
>
>    [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
>               in IPv6", RFC 3775, June 2004.
>
>    [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
>               Thubert, "Network Mobility (NEMO) Basic Support Protocol",
>               RFC 3963, January 2005.
>
>    [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
>               Gill, "IPv4 Multihoming Practices and Limitations",
>               RFC 4116, July 2005.
>
>    [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
>               (CIDR): The Internet Address Assignment and Aggregation
>               Plan", BCP 122, RFC 4632, August 2006.
>
>    [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
>               Workshop on Routing and Addressing", RFC 4984,
>               September 2007.
>
>    [1]  <http://www3.ietf.org/proceedings/06mar/slides/grow-3.pdf>
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>    [2]  <http://www.cidr-report.org/as2.0/, http://www.cidr-report.org/
>         cgi-bin/
>         plota?file=%2fvar%2fdata%2fbgp%2fas2.0%2fbgp%2das%2dcount%2etxt&
>         descr=Unique%20ASes&ylabel=Unique%20ASes&with=step,http://
>         www.potaroo.net/tools/asn32/>
>
>    [3]  <http://www.cidr-report.org/as2.0/>
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>    [4]  <http://www.potaroo.net/bgprpts/bgp-average-aspath-length.png>
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> Narten                   Expires August 16, 2010               [Page 24]
> 
> Internet-Draft   On the Scalability of Internet Routing    February 2010
>
>
> Author's Address
>
>    Thomas Narten
>    IBM
>
>    Email: narten@us.ibm.com
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> Narten                   Expires August 16, 2010               [Page 25]
> 
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>